The present invention is related to the improvement of the thermal aging stability of a structured automotive exhaust gas catalyst which is composed of a plurality of different catalytically active layers arranged above one another on a carrier, wherein the catalytically active layers comprise transition metals and porous support materials.
Catalysts for purifying the exhaust gases from internal combustion engines in motor vehicles, known as automotive catalysts, which are made up of various catalytically active layers arranged above one another on a carrier are well known and frequently have significant advantages in terms of their conversion behaviour and in particular in terms of their selectivity properties in the freshly produced state compared to catalyst formulations which are made up of the same type of catalytically active layers. The improved selectivity properties of these catalyst formulations are due to the exhaust gas to be purified having to come into contact in succession with a plurality of catalyst layers having different modes of action. As a result, the reaction paths of individual exhaust gas components can be controlled in a targeted manner and parallel reactions in which undesirable secondary emissions are produced can be prevented.
In particular, automotive exhaust gas catalysts which are to be used in motor vehicles having a predominantly stoichiometrically operated internal combustion engine, known as three-way catalysts, are typically made up of a plurality of different catalytically active coatings. These have been described in a very large number of patent applications and articles of the relevant technical literature.
For example, WO 95/35152 discloses a polyfunctional catalyst which is stable up to a temperature of 900° C. and simultaneously oxidizes hydrocarbons and carbon monoxide and reduces nitrogen oxides. This catalyst comprises a first, inner catalyst layer containing a first carrier material and a first palladium compound and optionally a first platinum group compound, at least one stabilizer and at least one compound of the rare earth metals and optionally a zirconium compound; and a second, outer catalyst layer containing a second support material, a second platinum compound, a rhodium component, oxygen storage components and optionally zirconium.
Furthermore, US 2005/0282701 describes a three-way catalyst which comprises a plurality of catalytically active layers and contains the palladium and platinum in the bottom layer applied directly to the carrier and rhodium and platinum in the second layer facing the exhaust gas. In addition, a “base coating” containing an oxygen-storage-material can be inserted between the carrier and the first layer in order to match the oxygen storage content of the overall catalyst to the appropriate vehicle application without changing the composition of the catalytically active coatings.
WO 06/044974 describes a catalyst composition which has a first layer containing a platinum component and a support material and on top of this a second layer containing a second support material and a sulphur oxide sorbent.
Structured automotive catalysts having various superposed, catalytically active layers have also been developed for other types of vehicles.
Thus, DE 198 54 794 describes a catalyst for the purification of the continuously lean exhaust gas of diesel engines which has a nitrogen oxide storage function in a first layer located directly on the carrier and whose second layer which is in contact with the exhaust gas contains a hydrocarbon storage function and catalytically active precious metals of the platinum group, wherein the platinum group metals are deposited on fine-grained, acidic support materials.
JP 2005-238199 describes an ammonia oxidation catalyst as can be used as ammonia barrier catalyst downstream of a catalyst for the selective catalytic reduction of nitrogen oxides in lean exhaust gases using ammonia as reducing agent in order to prevent the emission of excess ammonia. In this catalyst, a layer containing noble metal, which serves to oxidize ammonia, is inserted underneath a coating comprising titanium oxide, zirconium oxide, silicon oxide or aluminum oxide and a transition metal or a rare earth metal.
Unfortunately, structured catalysts which are made up of a plurality of superposed catalytically active layers and do not belong to the group of three-way catalysts, show a significantly larger decrease in conversion, based on the activity in the freshly produced state, and higher selectivity losses after thermal aging than it is the case for catalysts which contain uniform catalytically active coatings. This is a direct consequence of the heterogeneous structure of these structured catalysts.
The catalytically active coatings of such structured automotive exhaust gas catalysts generally comprise one or more transition metals which are applied in a highly dispersed form to a porous support material or are introduced into the pores of such a support material. These transition metals generally represent the reaction sites of the catalytic exhaust gas purification reactions.
For the purposes of the present text, the term “transition metals” encompasses the elements of the Periodic Table in which the d shell is being filled, i.e. all metals of the 4th, 5th and 6th periods which belong to the transition groups. Preference is given to using the metals of Groups VIII and IB in catalytically active coatings, in particular iron, copper and the noble metals ruthenium, rhodium, palladium, iridium, platinum, silver and gold. Excluded is technetium due to its undesirable radioactive properties.
As porous support materials for the transition metals, use is usually made of refractory oxides with a high surface area. Suitable support materials are, for example, γ-alumina, silica, aluminum silicates, zeolites, zirconia, titania, titanium-zirconium mixed oxides, the rare earths, in particular lanthanum oxide, ceria and cerium-zirconium mixed oxides, or mixtures thereof and also ternary oxides having a perovskite, hydrotalcite or spinel structure. An assumption for suitability as support material is, that the porosity of the material is sufficient for the application or introduction of transition metals. Accordingly, oxides which have a particle diameter from 1 to 30 microns and a specific surface area of at least 30 square metres per gram of support material (BET; determined in accordance with DIN 66132) are preferred. Particular preference is given to oxides having a specific surface area of from 50 to 300 square metres per gram of support material, in the case of zeolites from 300 to 800 square metres per gram of support material (BET; determined in accordance with DIN 66132).
Furthermore, there are transition metal oxides that show good catalytic properties and a sufficient porosity. These can be used directly as catalytically active coating without introduction of further transition metals. This applies, for example, to vanadium pentoxide, tungsten trioxide, titanium dioxide, cerium oxide and cerium-zirconium mixed oxides.
Catalytically active coatings which comprise one or more transition metals applied to or introduced into a porous support material or comprise a catalytically active transition metal oxide or comprise combinations thereof are typically applied in a thickness of from 15 to 150 microns per layer and a total layer thickness of from 30 to 300 microns to a carrier in an automotive catalyst. Carriers which can be used are ceramic honeycomb bodies, ceramic wall flow filter substrates, metallic flow-through bodies having parallel channels, ceramic foams and other flow-through substrates.
If a structured automotive catalyst which contains a plurality of superposed, catalytically active layers on a carrier is exposed to high temperatures over a relatively long period of time, adverse interactions between the layers can occur. For example, transition metal atoms which have been applied to the surface of the porous support material may thermally induced leave their adsorption positions and migrate to the interface to the adjacent catalytically active layer. If a sufficiently high concentration of the transition metal atoms is reached at the interface between two different catalytically active layers, they can pass through the interface into the neighbouring layer. The concentration at which such interface penetration occurs is referred to as the threshold concentration. The contamination of the neighbouring catalytically active layer resulting after the threshold concentration has been exceeded causes, that the separation of reaction sites which is usually typical for structured automotive catalysts is no longer being present, so that undesirable parallel reactions may occur. This results in undesirable reaction products and possibly pollutant gases which were not previously present, known as secondary emissions. The selectivity of the catalysts is greatly impaired and possibly destroyed.
For the purposes of the present text, secondary emissions are pollutant gases which do not result directly from the combustion process in the engine (primary emissions) but are produced by catalytic processes in the exhaust gas purification unit. Depending on the type of secondary emission resulting from the selectivity loss, this may also cause a significant decrease in the target conversion. If, for example, methane is formed by reduction of carbon dioxide or nitrogen oxide is formed by oxidation of ammonia used as reducing agent in an undesirable secondary reaction over an aged automobile exhaust gas catalyst, this is reflected in a decrease in the HC or NOx target conversion which can be measured over the overall exhaust system.
The decrease in the target conversion caused by the loss of activity and/or selectivity during operation at elevated temperatures is an indicator of the stability of the catalyst toward thermal aging processes for a person skilled in the art.